Amalickiah is second in command of the conjoined armies, a position which he might assert by his claimed royal birthright as well as a reward for putting such a large army under Lehonti’s control.
Poison has been traditionally popular throughout history for palace intrigues. Even though a claimant may have a valid right to a throne, using force to obtain that position is a costly way to obtain it, almost certainly creating enemies for the victor. Poison removes the occupant without condemning the aspirant. The ancient world had no scientific tests to prove death by poison, so while suspicions might be rife, evidence was not.
Amalickiah takes the least costly path to leadership of two armies: the over-king’s and Lehonti’s. Beyond his military capabilities, he also commands a significant group who can provide logistical support and, perhaps even more importantly, can support him as a legitimate claimant to a throne. Amalickiah finds some excuse to keep the army together. Perhaps they still saw themselves as a rebellious population and that another effort would be made to conquer them.